A radial tire for aircraft or for heavy duty use has at least one belt layer so as to maintain appropriate tread surface shape in severe conditions of high internal pressure and heavy load, and to endure the impact from the road surface. Each belt layer has a cord angle of 0 to 90 degrees with regard to the central circumferential line of the tire, and is formed by winding up a belt-shaped material cut into a required width, around a cylindrical belt drum or by winding it directly around a carcass swollen by an appropriate internal pressure.
In some cases, a belt layer having a wide sectional width is required in a section of the tire to maintain the necessary effects of the belt.
In such cases, if the shoulder portions of the belt layers are located along a carcass line in the forming and vulcanizing processes of a green tire, the inner diameter of the belt at the shoulder portions becomes shorter than the inner diameter at the center portion.
When a tire having such a belt structure is filled with air, if belt plies are composed of one-piece structures respectively, an uneven tension is applied on the belt plies. That is, the tension at the tread shoulder portions becomes smaller than the tension at the tread center portion because of the difference in the above-mentioned inner diameters.
This fact might become a cause of serious structural breakage especially in radial tires mounted on an aircraft which lands and takes off at high speed. For example pneumatic radial tires attached to an aircraft taking off at a speed of about 370 km/h reach that speed in a short time under heavy load. At this moment, in the shoulder portions where the belt tension is relatively small, a phenomenon in which the belt edges are deformed outward in the radial direction of the tire, so-called lifting, occurs due to the centrifugal force added to the tread. It induces an increase in distortion and causes separation.
A method employed to prevent such a problem is to construct the belt ply by a so-called cordwinding method in full width so as to unify the tension applied on the belt cord at a high running speed, but this method is poor in production efficiency. Another method is to design a profile having a constant belt diameter, but in tires used at a high internal pressure, such as aircraft tires, the increase in the diameter when filled with air is so large at the tread center that the tension applied on the tread center is forced to be large.
It is hence a primary object of the present invention, to unify the tension applied to the belt cord when filled with air without reducing the efficiency in production, to decrease lifting at the belt edges at high speed, and therefore to enhance the structural durability of the pneumatic radial tire at high speed.